Mycobacterium tuberculosis is currently the etiologic agent that is responsible for the most deaths worldwide. Mycobacterium avium is the most common cause of systemic bacterial infection in American AIDS patients. Despite the public health importance of these diseases, the mechanisms of virulence remain unknown. We have previously shown that the catalase-peroxidose (katG) protein is a M. tuberculosis virulence factor. We have also demonstrated that genes linked to katG are involved in virulence. We are currently evaluating the role of genes adjacent to katG in the virulence of M. tuberculosis.